For women with hormone receptor-positive breast cancer, long-term use of adjuvant endocrine therapy (AET) significantly reduces the risk of hospitalizations, cancer recurrence and mortality, and increases quality of life. Despite the known benefits of AETs, many patients are nonadherent due to adverse side effects. Furthermore, lower adherence among black women may be contributing to the large and growing disparities in mortality outcomes. Real-time monitoring of treatment-related adverse symptoms and adherence could result in more effective management of symptoms, higher medication adherence, and ultimately lower recurrence and mortality. To date, however, only a few interventions have aimed to improve AET adherence, few have targeted symptom management as a means to improve adherence, and of these, none have found a statistically significant impact on adherence. Our proposed study will fill this research gap by testing a web- enabled app designed with the explicit goal of improving long-term AET adherence. Patient-reported symptoms will be integrated directly with the patient's electronic health record in order to improve patient-provider communication about AET adherence and related adverse symptoms outside of clinic visits. In a small pilot trial of the study app, we found that participants who had recently initiated a new AET and received weekly reminders to use the app reported significantly higher adherence to AETs at 8 weeks compared with a control group (91% vs. 68%, p=0.02). The proposed study builds on the success of our pilot study by: 1) expanding the intervention period to six months in order to capture later-onset adverse symptoms that are slower to develop; 2) following participants for one to three years, depending on enrollment year, to test longer-term effects of the intervention on medication adherence and other outcomes; and 3) including a larger sample powered to test multiple levels of the intervention. We will randomize 300 participants to one of three arms: 1) an ?App? group (n=100) that will receive weekly reminders to use the PCM app; 2) an ?App+Feedback? group (n=100) that will receive weekly reminders and personalized feedback based on their use of the app; or 3) a ?Usual Care? group (n=100) that will receive usual care only. The app will include questions about AET adherence and adverse symptoms with built-in alerts sent to the patient's care team if any concerning symptoms or trends are reported. We hypothesize that monitoring symptoms and adherence with actionable alerts and tailored feedback reports to patients will result in timelier symptom management and higher long- term adherence to AET. By evaluating the impact of the intervention on a comprehensive set of measures, including AET adherence, patient outcomes, racial disparities and resource use-related costs, our study will provide valuable and actionable results for providers, policy makers, and insurers who strive to achieve the ?Triple Aim? ? reduce costs while improving health outcomes and the patient experience.